Organic light emitting diodes (OLEDs) suffer from a relatively low efficiency of emitted light relative to that generated because of light trapping due to a refractive index mismatch between the air (n=1), glass substrate (n=1.52), and an electrode/active layer, for example, an ITO/organic layer (˜1.7 to ˜2.0) in waveguide modes. The imposition of a periodic microstructure into an OLED permits the recovery of a portion of the lost light. Unfortunately, although improvement of the light output has occurred, the cost to achieve the periodic microstructure is that of complicating the process with lithographic and imprinting steps to prepare and transfer the microstructure to the OLED.
Bowden et al., Nature 1998, 393, 146-9 demonstrated the preparation of buckled structure by the deposition of metal films on a thermally expanded 1 cm thick polydimethylsiloxane (PDMS) rubber film, where upon subsequent cooling of the films, buckling of the metal film results from the compressive stress imposed by the cooling rubber where the buckles display a uniform wavelength of 20-50 micrometers. The buckles had periodicities between 20 and 50 μm; with a depth from the crest to the trough of 1.5 μm for metal deposition performed without external heating, where the temperature imposed by the hot evaporating metal appeared to result in a surface temperature of about 110° C., and depths of 3.9 μm for deposition conducted with external heating to 300° C. Regularly buckled semiconductor ribbons were disclosed by Khang et al., Science 2006, 311, 208-12 and Jiang et al., Applied Physics Letters 2007, 90, 133119, where a PDMS rubber is deformed along a single axis resulting in a periodically buckled semiconductor ribbon with periodic lengths in excess of a micron. Yu et al., Applied Physics Letters 2010, 96, 041111 demonstrated regular buckled patterns with submicron periodicity by controlling the thickness of the metal films to about 10 nm on 1 mm thick PDMS rubber strips that were pre-stretched along one axis at a desired pre-strain.
Koo et al., Nature Photonics 2010, 4, 222-6 extended the use of buckled metal surfaces to the preparation of OLEDs. PDMS was cured at 100° C. and an aluminum layer was deposited on the rubber at a thickness of about 10 nm. Upon cooling, a quasi-periodic buckled surface formed and was used to mold a PDMS replica of the buckled surface, which is subsequently metallated with Al and used to form a UV cured resin replica of the PDMS replica, where the final resin replica is used as a master template. The template was used to form a second PDMS replica after Al deposition, followed by forming a second resin replica, which, upon removal of the PDMS replica, is UV-ozone treated and used as a substrate that is sputtered with an ITO glass to form an anode. Subsequently an OLED is formed upon deposition of a hole transport layer, a light emission-electron transport layer, an electron injection layer, and an Al cathode layer. This OLED had an enhancement of the emitted intensity of about 2.2 fold at 525 nm and about fourfold at 655 nm.
There remains a need to achieve a periodic or a quasi-periodic microstructure that does not require a significant departure from existing fabrication techniques of an OLED. Particularly desirable is a quasi-periodic structure where the observed efficiency of the light output is improved by the relationship of the emission wavelength to the polar angles and azimuthal angles of the quasi-periodic structure and in a manner where the light output is not blurred because of the buckled structure.